1. Field of the Invention
The present invention is related to optical transmission methods, error rate evaluation methods and signal generating apparatuses, and more specifically to an error rate evaluation method of a quadrature amplitude modulation (QAM) signal (hereinafter referred to as QAM signal) when a television video signal obtained by vestigial-sideband amplitude modulation (hereinafter referred to as AM video signal) and a QAM signal are multiplexed and optically-transmitted, a signal generating apparatus for generating a test signal for use in the above evaluation method and a transmission method for optically transmitting the multiplex signal.
2. Description of the Background Art
Hereinafter referred to as a first document is "QAM Transmission Performance in AM/QAM Hybrid SCM Optical Transmission System" by Maeda et al., 1991 Television Society national convention preliminary manuscript collection, pp. 473-474.
Further, referred to as a second document is "Analysis of BER of 16QAM signal in AM/16 QAM hybrid optical transmission system" by K. Maeda et al., Electronics Letters, Vol. 29, PP. 640-642 (1993).
Further, referred to as a third document is "Broad-Band AM-VSB/64QAM Cable TV System over Hybrid Fiber/Coax Network" by X. Lu et al., IEEE Photonics Technology Letters, Vol. 7, No. 4, April 1995.
Still further, referred to as a fourth document is "The Effect of Analog Video Modulation on Laser Clipping Noise in Optical Video-Distribution Networks" by Stuart S. Wagner et al., IEEE Photonics Technology Letters, Vol. 8, No. 2, February 1996.
Conventionally, as a method for multiplexing and optically-transmitting a multiplex signal of AM video signals and a QAM signal in a video transmission system, a method described in the first document has been suggested. The method described in the first document is to multiplex an AM video signal with QAM signals intended for distribution of digital compression video signals and the like in a cable television (CATV) system.
The second document analyzes the transmission characteristics of the QAM signal in the optical transmission method and the like described in the first document. In the second document, described is a method for evaluating the error rate characteristic of the QAM signal by optically transmitting a test signal obtained by multiplexing a carrier approximate to the AM video signal and the QAM signal.
Specifically, reported in the second document are that the transmission quality of QAM signals is extraordinarily deteriorated by clipping distortion which occurs in a semiconductor laser due to AM video signals, and that clipping distortion of AM video signals is predominant over the transmission quality of QAM signal.
Conventional apparatuses for use in evaluating the error rate characteristic of QAM signals described in the second document include, for example, a TV multi-channel signal generator in Panasonic electronics measuring instrument catalog, 1994 edition, p. 219 (hereinafter referred to as a conventional multiplex signal generator). The conventional multiplex signal generator generates a carrier approximate to an AM video signal or generates a test signal obtained by modulating the carrier with a pulse signal, evaluating complex mutual modulation distortion, cross modulation interference and the like in an amplifier used in a transmission path of a CATV system and a television signal transmission apparatus such as an optical transmission apparatus.
Described briefly below is the above conventional multiplex signal generator referring to FIG. 9.
FIG. 9 is a block diagram showing an example of the structure of the conventional multiplex signal generator. In FIG. 9, the conventional multiplex signal generator includes a plurality of signal sources 61.sub.1 to 61.sub.N (N is an integer not less than 2), a pulse signal generating portion 631, a multiplex portion 641, and a pulse modulation portion 621.
The plurality of signal sources 61.sub.1 to 61.sub.N are so-called oscillators, each generating an independent carrier with a different frequency (the total number of generated carriers is N). The pulse signal generating portion 631 generates a pulse signal having a period of 15.75 kHz, which is a horizontal synchronizing period of a color television signal in the present NTSC system, and a pulse width of its horizontal synchronizing signal (=0.08* (horizontal synchronizing signal period) .mu.sec). The multiplex portion 641 multiplexes the carriers outputted from the plurality of signal sources 61.sub.1 to 61.sub.N and outputs a resultant multiplex signal. When cross modulation distortions are evaluated, the pulse modulation portion 621 receives the inputs of the above multiplex carrier and the above pulse signal, and then outputs a signal obtained by modulating the carrier with the pulse signal.
On the other hand, described in the third document is an optimal optical modulation index of the AM video signal in the optical transmission method of simultaneously transmitting the AM video signal and the QAM signal shown in the first document. More specifically, when the AM video signal of 60 channels and the 64QAM signal are optically transmitted simultaneously, the effect of clipping is negligible due to the optical modulation index of the AM video signal being set to not more than 3.7%. The third document reports that this allows high-quality optical transmission of 64QAM signals.
However, in recent years, new findings have been obtained against the view in the second document as to evaluation of QAM signal transmission quality in the method of multiplexing and optically transmitting an AM video signal and a QAM signal described in the second document. For example, the following characteristics are described in the fourth document.
In actual video communications systems, the AM video signal is modulated with a video signal, and the AM video signal thus has an amplitude smaller than that of the test signal generated in the above conventional multiplex signal generator, and clipping distortion in a semiconductor laser occurs less. However, the above conventional multiplex signal generator generates a plurality of carriers and multiplexes these carriers to output as a test signal. Therefore, the amplitude of the test signal has a wide range in normal distribution than the amplitude of the actual AM video signal, resulting in a higher probability of clipping distortion in the laser. Further, a pulse-modulated signal obtained by use of a pulse modulation function of the above conventional multiplex signal generator (the pulse modulation portion 621 and the pulse signal generator 631) is obtained by modulating a multiplex carrier with one pulse signal in the final output stage. Therefore, when only a pulse occurring period is considered, the result is also the same when only a carrier without pulse modulation is multiplexed, and error rate deterioration due to clipping is approximately the same (refer to FIG. 10).
Therefore, the value of the error rate deterioration of the QAM signal in the actual AM video signal is lower than that when evaluated by the test signal of the above conventional multiplex signal generator. The evaluating conditions are thus more stringent when the error rate of the QAM signal is evaluated with the test signal generated in the above conventional multiplex signal generator compared with when the error rate is evaluated in the actual video communications system, resulting in accurate error rate evaluation impossible. Furthermore, the fourth document does not mention the optimal distribution of each modulation index of the AM video signal and the 64 QAM signal when modulation signals are multiplexed as in this actual system.